A slightly more dramatic demonstration of buoyancy than simply having a series of balls float in water is to float soap bubbles on a layer of carbon dioxide.
- Fish tank
- Large bag of baking soda
- Gallon jug of vinegar
- Soap bubble blower
- Pour the baking soda into the fish tank and then pour in the vinegar. It will bubble vigorously creating the CO2 layer. You need quite a lot of each.
- Wait for all the bubbling to stop. You may need to mix it up a bit, but not too much so that you do not mix up the CO2 layer too much.
- Blow the bubbles into the fish tank. If it all worked out then the soap bubbles should appear to float in mid-air.
This is a largely qualitative demonstration, though you can make up some numbers to do a simple calculation. Take a 1 cm radius soap bubble floating in the CO2 layer.
If you assume that the bottom half of the bubble is in the CO2 layer and the top half is in the air then you can calculate the buoyancy force acting on the bubble due to the CO2 and the air
FB= (4/3)πr3 ((½)ρCO2 g + (½)ρair g)
This is balanced by the weight of the air in the bubble plus the weight of the soap.
W= (4/3)πr3 ρair g + 4πr2Tρsoapg
where T is the thickness of the soap film. The balance then becomes
FB= W or (4/3)πr3 (½ρCO2 g + ½ρair g)= (4/3)πr3 ρair g+ 4πr2Tρsoapg or (½)r (ρCO2 + ρair )=r ρair + 3Tρsoap
which leads to
T=(½)r( ρCO2 – ρair)/3ρsoap
Substituting material properties (ρCO2=1.98 kg/m3 ρair=1.23 kg/m3 and ρsoap=900 kg/m3 ) into the equation give T=1.4 μm. This is consistent (at least in order of magnitude) with thin-film Interference estimates.